What is spin?

الأصل · EN

This is a late answer to question #79 by R.I. Khrapko, "Does plane wave not carry a spin?," Am. J. Phys. /69/, 405 (2001), and a complement (on gauge invariance, massive spin 1 and 1/2, and massless spin 2 fields) to the paper by H.C. Ohanian, "What is spin?," Am. J. Phys. /54/, 500--505 (1985). In particular, it is confirmed that "spin" is a classical quantity which can be calculated for any field using its definition, namely that it is just the non-local part of the conserved angular momentum. This leads to explicit expressions which are bilinear in the fields and which agree with their standard "quantum" counterparts.